The present invention relates to a sensor for measuring the rotational speed of a turboshaft of a turbocharger with a sensor housing and a sensor element positioned in the sensor housing and which senses a variation of a magnetic field caused by the rotation of the turboshaft.
The power generated by an internal combustion engine depends on the air mass and the corresponding amount of fuel that can be made available to the engine for combustion. If an increase in the power of the internal combustion engine is desired, an increased amount of combustion air and fuel must be supplied to the engine. In a naturally aspirated engine, this increase in power is achieved by an increase in the swept volume or by increasing the rotational speed. However, an increase in the swept volume leads in principle to engines that are heavier, have larger dimensions, and are consequently more expensive. Increasing the rotational speed entails considerable problems, in particular in the case of larger engines, and is limited for technical reasons.
A much used technical solution for increasing the power of an internal combustion engine is that of supercharging. This is the term used for boosting the combustion air by an exhaust gas turbocharger or else by means of a compressor that is mechanically driven by the engine. An exhaust gas turbocharger substantially comprises a flow compressor and a turbine, which are connected to a common shaft and rotate at the same speed. The turbine converts the normally wasted energy of the exhaust gas into rotational energy and drives the compressor. The compressor sucks in fresh air, compresses it and transports the compressed air to the individual cylinders of the engine. The greater amount of air in the cylinders can be supplied with an increased amount of fuel, whereby the internal combustion engine delivers more power. The combustion process is also favorably influenced, so that the engine achieves a better overall efficiency. In addition, the torque characteristic of an internal combustion engine supercharged by a turbocharger can be formed extremely favorably. By using an exhaust gas turbocharger, normally aspirated production engines that vehicle manufacturers have in stock can be significantly optimized without major modifications of the structural design of the engine. Supercharged internal combustion engines generally have a lower specific fuel consumption and have lower pollutant emission. In addition, turbo-engines are generally quieter than naturally aspirated engines of the same power, since the exhaust gas turbocharger itself acts like an additional muffler. In the case of internal combustion engines with a great operating speed range, for example in the case of engines for passenger cars, a high boost pressure is required even at low engine speeds. In the case of these turbochargers, a boost-pressure control valve, known as a waste-gate valve, is introduced. By choosing an appropriate turbine housing, a high boost pressure is quickly built up even at low engine speeds. Once the appropriate high boost pressure is reached, the boost-pressure control valve (waste-gate valve) then limits the boost pressure to a constant value as the engine speed increases. As an alternative to this, turbochargers with variable turbine geometry (VTG) are used. In the case of these turbochargers, the boost pressure is regulated by changing the turbine geometry.
With an increasing amount of exhaust gas, the maximum permissible rotational speed of the combination comprising the turbine wheel, the compressor wheel and the turboshaft, also referred to as the running gear of the turbocharger, may be exceeded. Inadmissible exceeding of the rotational speed of the running gear would cause it to be destroyed, which would be equivalent to total loss of the turbocharger. Modern turbochargers are built with much smaller turbine wheel and compressor wheel diameters and have improved rotational acceleration behavior as a result of a considerably smaller mass moment of inertia. However, these types of turbochargers are particularly susceptible to damage caused by exceeding the permissible maximum speed. Depending on the design of the turbocharger, even exceeding the speed limit by approximately 5% leads to complete destruction of the turbocharger.
The boost-pressure control valves, which according to the prior art are controlled by a signal resulting from the generated boost pressure, have been successfully used for speed limitation. If the boost pressure exceeds a predetermined threshold value, the boost-pressure control valve opens and conducts part of the mass flow of exhaust gas past the turbine. As a result of the reduced mass flow, said turbine takes up less power, and the compressor power is reduced to the same degree. The boost pressure and the rotational speed of the turbine wheel and of the compressor wheel are reduced. However, this control is relatively sluggish, since the pressure buildup when the rotational speed of the running gear is exceeded takes place with a time delay. Therefore, the speed control for the turbocharger with the boost pressure monitoring must intervene in the highly dynamic range (power-off) by appropriately early reduction of the boost pressure, which leads to a loss of efficiency.
Direct measurement of the rotational speed at the compressor wheel or at the turbine wheel proves to be difficult, since for example the turbine wheel is exposed to extreme thermal loading (up to 1000° C.), which prevents speed measurement by conventional methods at the turbine wheel. In a publication by acam-Mess-elektronic GmbH of April, 2001, it is proposed to measure the compressor blade pulses on the eddy current principle and determine the speed of the compressor wheel in this way. This method is complex and expensive, since at least one eddy current sensor would have to be integrated in the housing of the compressor in the direct vicinity of the compressor blades, which is likely to be extremely difficult because of the high precision with which components of a turbocharger are produced. Apart from the precise integration of the eddy current sensor in the compressor housing, sealing problems arise, problems which, on account of the high thermal loading of a turbocharger, can only be overcome by sophisticated modifications of the structural design of the turbocharger.